From what science has proven till now, it is not possible to just turn into a fairy-tale character. Which means if you find a spell that allows you to physically change into a mermaid, werewolf, vampire, etc, it will not work and you shouldn't waste your time on it.
If anyone claims to be any of the above, In the "physical form", do not believe them. You can ask for proof, you will see that they are suddenly busy and have to leave!
To be honest even if they existed, they wouldn't waste their time trying to tell others, they would keep it a secret and enjoy being unique.
There ARE spiritual paths such as the Werewolf path and Vampyrism, but they are totally different from being a role player. As far as I can remember even the werewolf path calls some people "Pseudo Werewolves". Pseudo Werewolves are those who claim that they can physically shape-shift into a wolf. Same goes with Vampyrism, vampire fluffs are not allowed there.
And again, you can not change your DNA with a spell. Do not trust whatever you see there. Most of them don't even know what magick means.